What is your guys favorite instrument to use when recording ambient music?

Omnisphere, but then what does that say... as in: what genre would it not work for?

Then, again, even I can cobble together some nice ambient drone sounds using Bazille, Blue2, Serum, Diva, Predator, Thor, and probably Massive. I read a quote recently on photographic cameras: the best camera is the one you have with you. Analog waveforms, chords, LFO, and resonant filter seems like most of what you need (depending on sub-genre, people can get so finicky these days). And a sampler. Kontakt is the way to go there, which technically includes Exhale, as well as Heavyocity as they run within that platform.